Advanced techniques for characterization of heterogeneous catalysts

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1 Graduate School of Materials Research (GSMR) Graduate School in Chemical Engineering (GSCE) Department of Chemical Engineering (ÅA) Advanced techniques for characterization of heterogeneous s (4 credits/sp/op/bologna) Main lecturer: Prof. Andrey Simakov Centro de Nanociencias y Nanotecnologia Universidad Nacional Autonoma de Mexico Lecturers: Prof. Tapio Salmi, Åbo Akademi Prof. Dmitry Murzin, Åbo Akademi Transient analysis of catalytic reactions including in situ and operando spectroscopic measurements

2 In general transient method means that one or more parameters of the system are perturbed or varied and certain kinds of responses are measured. The transient methods can be classified into two main groups: the first one is when due to the perturbation the system has been transformed into another thermodynamic state. (For example the classical Temperature Programmed Desorption and concentration jump methods belong to this group. ) The second group is when the system remains in the same thermodynamic state during transient experiment. (Literally this means different kinds of labeling techniques.) Advanced techniques for characterization of heterogeneous s The transient response techniques applied to heterogeneous catalysis have been reviewed by: Kobayashi, H., Kobayashi, M.,Transient response method in heterogeneous catalysis, Catal. Rev.. Sci. Eng. 10 (1974) Furusawa,T., Suzuki, M., Smith, J. M., Rate parameters in heterogeneous catalysis by pulse techniques, Catal. Rev.. Sci. Eng. 13 (1976) Bennett, C. O., The transient method and elementary steps in heterogeneous catalysis, Catal. Rev.. Sci. Eng. 13 (1976) Bennett, C. O., Experiments and processes in the transient regime for heterogeneous catalysis, Adv. Catal. 44 (2002) Tamaru, K., Dynamic Heterogeneous Catalysis, Academic Press, New York, Mirodatos, C., Use of isotopic transient kinetics in heterogeneous catalysis, Catal.Today 9 (1991) Advanced techniques for characterization of heterogeneous s

3 The examples of transients combined with IN SITU or operando measurements Transient analysis of CO oxidation over Au-TiO 2 using operando DRIFTS and MS

4 Motivation To probe aspects of the reaction pathways including desorption, oxygen storage, reaction rates and the role of carbonates during CO oxidation reactions using supported Au nanoparticles (3.7 nm) as a. *[Catalysis Today 126 (2007) ] During the transients the FTIR spectrum of the pellet and the composition of the gas flowing out of the reactor were continuously monitored. Advanced techniques for characterization of heterogeneous s

5 Home made FTIR cell for fast transients In order to observe fast surface transients, it is required that the reactor has a low dead volume, and a reactor was constructed with this as a major design criterion. A cylindrical, CaF 2 rod (A) slides through internal O-rings located on both halves. A sample pellet is held between the CaF 2 rods when the two halves are bolted together. Gas flows into and out of the cell through small holes (B) and is contained within the volume sealed by the O-ring (C). Chilling fluid is circulated through ports in the body of both halves (D). The reactor volume surrounding the wafer is theoretically <0.05 cm3!!! Analysis of cell transient parameters Ar He The normalized Ar concentration profile, c(t), through the bypass can be fit well to an equation which contains an exponential decrease combined with a diffusive component in the source term: σ = 2.1 s, V/F = 0.31 s where σ is a parameter describing the mixing at the interface following the gas switch, V cell volume, F gas flow. For the proposed reactor configuration most of a non-interacting gas can be eluted from the reaction cell in less than a few seconds (!!!), the time resolution limit of the mass spectrometer sampling system.

6 Switches He CO+O 2 +He+Ar He at T= -10 o C MS transient CO a Carbonates-bicarbonates CO 2 (a) FTIR transient Spectral features of CO adsorption-desorption desorption Due to competing interactions CO was adsorbed on metallic Au nanoparticles

7 Kinetics of CO desorption Because area under the peak Is promotional to the concentration it s possible to calculate changes of surface coverage for different temperatures Heats of adsorption measured from CO adsorption isotherms on Au(110) surface is 7.8 ± 1.3 kcal/mole So, there are the following features: the close correspondence with Au single crystal data for the FTIR peak positions; the coverage dependent shifts; The desorption/adsorption energies together. These features indicate that the CO adsorption is occurring on the Au particles and not the support. CO 2 does not enhance CO desorption.

8 Difference in adsorption and desorption difference in peak evolution The different peak evolutions for adsorption compared to desorption indicates that these two processes are governed by different dynamics. This result can be explained by a clustering model where increasing coverage of CO(a) first adsorbs on clean Au particles in widely spaced atop sites (2116 cm -1 ), followed by nucleation of denser CO(a) islands (2106 cm -1 ) which grow (with frequency shift) and dominate at higher coverage. Desorption (or reaction) occurs primarily from island edges causing a gradual frequency shift as the islands shrink. Apparently, clustering ofco and/or the distribution of the two different CO(a) species (island versus isolated) is not at equilibrium but depends upon whether the sites are populating or depopulating. Storage of CO and reactive oxygen Now we ll consider the relative rate of desorption of surface CO compared to the rate of its reaction on the surface. Such information can be obtained by conducting a direct switch or a delayed switch from CO to O 2. In a direct switch, CO is flowed for a period of time and then the gas stream is switched abruptly to O 2 and reaction of surface CO(a) begins. CO O In a delayed switch, the CO flow is switched to He, which flows for a timed interval before O 2 is switched into the reactor. CO(a) desorbs during the He interval and then reaction commences when the oxygen reaches the reactor zone. CO He O 2

9 Transient data for CO desorption at 248 K the reaction rate of CO(a) is competitive with or faster than the desorption rate. CO may be stored briefly on the surface when oxygen is not present. FTIR peak located near 2105 cm -1 is shown as a function of time after a direct or delayed switch to O 2 Subtracting the observed desorption rate yields a reaction rate of about s -1 which can be equated with a TOF. It is comparable with the TOF measured in a flow reactor being equal to s -1 at 235 K. Oxygen storage Although adsorbed oxygen cannot be observed on the surface by FTIR, its presence may be detected indirectly by its effect on CO uptake. The actual delay of less than 2 s implies that either less than 3.1 µmoles O/g-catal is stored anywhere on the surface or the oxygen desorption rate at 298 K is rapid compared to the CO arrival rate. O 2 is flowed for 60 s prior to a direct switch to CO Note, that there is 1.0 µmoles of surface Au

10 Surface carbonate species Similar spectra both for Au- TiO 2 and TiO 2 support! Desorption of CO 2 The position of this peak, assigned to adsorbed CO 2 (a) is independent of coverage, and occurs at the same frequency when the Au-free TiO 2 is used in the reactor.

11 Desorption of CO 2 Desorption of the CO 2 (a) is slower than for CO at comparable temperatures and appears to have both a fast and a slow component of desorption. FAST - a very low activation energy of around 1 2 kcal/mole, which is characteristic for diffusion controlled removal process. The similar peak positions, intensities and activation energies for TiO 2 and Au-TiO 2 suggest that the CO 2 (a) is primarily present on the support. Desorption of CO 2. Interconnection with surface carbonates Transients after switch CO/O 2 to He at -20 o C Fast response desorption of adsorbed CO 2. Slow response decomposition of carbonates.

12 Pathways for CO oxidation on Au-TiO 2 Adsorption of O 2 on WGC either occurs at a very low coverage, or its desorption is very fast. CO adsorbs on Au nanoparticles readily. CO(a) reacts rapidly in the presence of gas phase O 2 to form CO 2 (a). The product CO 2 can then desorb from the Au particle and interact with the support to form transient carbonate species which then slowly decompose to CO 2. The desorption of CO 2 (a) is the rate limiting step in CO oxidation over Au-TiO 2. Study the storage and reduction of NO X over Pt(or Pd or Rh)/BaCO 3 /Al 2 O 3

13 Study the storage and reduction of NO X over Pt(or Pd or Rh)/BaCO 3 /Al 2 O 3 Motivatiion NO X storage and reduction technology offers the possibility of reducing emissions of NO X from vehicles operating under lean-burn conditions. The concept is based on incorporating a storage material (commonly Ba) in the conventional three-way to store NO X (NO + NO 2 ) under lean conditions until it is saturated with NO X. Subsequently the stored NO X is released and reduced to N 2 by turning the engine to rich operating conditions under a short period. The concept was introduced by Toyota in the beginning of the 1990s. NO + NO 2 R NO X N 2 R = CO, H 2, C 3 H 6, or C 3 H 8 NO X storage and reduction cycles for Pt/BaCO 3 /Al 2 O 3 CO Gas phase analysis H 2 lean rich lean rich lean rich The efficiency for NOx reduction changes with T and reducing agent. CO can block some Pt sites and decrease efficiency of reduction.

14 The in situ DRIFTS experiments were performed using a BioRad FTS6000 FTIR spectrometer equipped with DRIFTS optics and a heated reaction chamber (Harrick Scientific Praying Mantis with a DRIFTS cell). Experimental conditions Temperatures: 350, 250,and 150 C Total flow rate: 200 ml/min, Space velocity: 106,000 h 1. Experimental steps The pretreated was first saturated with NO X by exposing it to 500 ppm NO 2 in Ar for 19 min. The stored NO X was subsequently reduced by introducing a reducing agent to the NO 2 /Ar flow. The regeneration gas mixture consisted of 500 ppm NO 2 and 4000 ppm CO, 4000 ppm H2. Two consecutive storage reduction (lean rich) cycles were conducted to follow the evolution of the surface species under the second cycle. The spectra were collected during the entire NO X storage reduction cycles with a time resolution of 0.5 s. monodentate nitrate over BaO monodentate nitrite over BaO monodentate nitrite over Al 2 O 3 bidentate nitrate over Al 2 O 3 bridge-bonded bidentate nitrite over BaO In Situ DRIFTS spectra Regeneration by CO NO X storage occurs via the formation of nitrites and nitrates of both barium and alumina. With increasing temperature, NO X storage on barium becomes more significant.

15 An operando UV vis spectroscopic study of the catalytic decomposition of NO and N 2 O over Cu-ZSM ZSM-5* An operando UV vis spectroscopic study of the catalytic decomposition of NO and N 2 O over Cu-ZSM-5* Motivation operando UV vis spectroscopy in combination with on-line GC analysis allows a direct study of the role of the bis(μ- xo)dicopper core in the catalytic NO and N 2 O decomposition cycles 2 NO N 2 + O 2 N 2 O N 2 + ½ O 2 The decomposition of NO is thermodynamically favored at temperatures below 1000 K. However, the reaction is kinetically retarded due to the very high activation energy of about 300 kj mol 1. *[Journal of Catalysis 220 (2003) ]

16 An operando UV vis spectroscopic study of the catalytic decomposition of NO and N 2 O over Cu-ZSM-5* Much research has been dedicated to the identification of the active sites in Cu- ZSM-5 and the NO decomposition reaction mechanism. WHY? A 100% selective conversion of NO into N 2 and O 2 can be achieved over Cu-ZSM- 5 above 350 o C with maximum activity in the temperature range o C. Several explanations for this profile have been proposed: the desorption of oxygen, the instability or surface nitrates, the adsorption equilibrium of NO. Different active species were proposed: Cu(I) and Cu(II) ELO species (ELO, extralattice oxygen); Mononuclear species Cu 2+ O or Cu 2+ O 2 ; [CuOCu] 2+ ; According to XAFS An operando UV vis spectroscopic study of the catalytic decomposition of NO and N 2 O over Cu-ZSM-5 Experimental set-up

17 An operando UV vis spectroscopic study of the catalytic decomposition of NO and N 2 O over Cu-ZSM-5 O Cu Cu O O zeolite Cu(II) CT transitions d d transitions of (partially hydrated) Cu(II) ions The evolution of the in situ UV vis spectra of Cu-ZSM-5 collected during calcination in oxygen flow. An operando UV vis spectroscopic study of the catalytic decomposition of NO and N 2 O over Cu-ZSM-5 Transient response after switch O 2 NO They can be assigned to a single Cu-ELO species

18 An operando UV vis spectroscopic study of the catalytic decomposition of NO and N 2 O over Cu-ZSM-5 Time dependence of the NO decomposition An operando UV vis spectroscopic study of the catalytic decomposition of NO and N 2 O over Cu-ZSM-5* Proposed reaction mechanism for the decomposition of NO and N 2 O

19 Transient Technique for Identification of True Reaction Intermediates: Hydroperoxide Species in Propylene Epoxidation on Gold/Titanosilicate icate Catalysts by X-ray X Absorption Fine Structure Spectroscopy Transient Technique for Identification of True Reaction Intermediates: Hydroperoxide Species in Propylene Epoxidation on Gold/Titanosilicate Catalysts by X-ray Absorption Fine Structure Spectroscopy Motivation Propylene oxide (PO) is a well-known chemical intermediate used in the synthesis of polyether polyols, propylene glycols, and propylene glycol ethers, which are subsequently employed in the preparation of many end products such as polyurethanes, cosmetics, hydraulic, antifreeze and brake fluids, coatings, inks, textile dyes, and solvents. Because of the high selectivity to PO (>90%) and the lower cost of the feedstocks, the hydrogen-oxygen route over Au/Ti-SiO 2 s has attracted great attention.

20 Possible sequence of steps for propylene epoxidation with H 2 and O 2 on Au-supported titanosilicates The important steps during PO synthesis consist of: synthesis of hydrogen peroxide from hydrogen and oxygen on gold nanoparticles; formation of Ti-hydroperoxo or peroxo species from hydrogen peroxide on tetrahedral Ti centers; reaction of propylene with the Ti-hydroperoxide species to form PO; decomposition of hydrogen peroxide to water. Transient Technique for Identification of True Reaction Intermediates: Hydroperoxide Species in Propylene Epoxidation on Gold/Titanosilicate Catalysts by X-ray Absorption Fine Structure Spectroscopy Techniques to be used: In situ transient UV-vis data are used to identify the formation of Tihydroperoxide species and to verify its characteristic behavior as a reaction intermediate. In situ XAFS spectroscopy is another powerful technique that can be used to obtain atom-specific structural and electronic information from solid inorganic s under operating conditions. Short presentation of XAFS technique

21 In situ UV-vis spectra for Au-Ba/Ti Ba/Ti-O under H 2 /O 2 /Ar (1/1/8) gas mixture at 150 o C 3.82 ev - Ti-hydroperoxo species, 3.96 ev (313 nm) and 4.18 ev (297 nm) - water coordinated to Ti sites. In situ UV-vis spectroscopy results for Au-Ba/Ti Ba/Ti-O under propylene epoxidation conditions (C 3 H 6 /H 2 /O 2 /Ar ) 1/1/1/7) at 150 o C 3.82 ev - Ti-hydroperoxo species, 3.96 ev (313 nm) and 4.18 ev (297 nm) - water coordinated to Ti sites.

22 Intensity of band corresponding to titanium peroxide vs time in flow H 2 /O 2 /Ar = 1/1/8 C 3 H 6 /H 2 /O 2 /Ar = 1/1/1/7 Intensity of band corresponding to titanium peroxide vs time after different switches

23 In situ Ti K-edge K XANES spectroscopy data as a function of time in flow of reaction mixture 4-fold coordinated Ti (C 3 H 6 /H 2 /O 2 /Ar = 1/1/1/7) Rates of formation of 4-fold 4 coordinated Ti under the treatment in the different gases H 2 /O 2 /Ar = 1/1/8 C 3 H 6 /H 2 /O 2 /Ar = 1/1/1/7

24 So, measurement of the changes in Ti-hydroperoxo coverage under transient experiments at reaction conditions with H 2 /O 2 /Ar and C 3 H 6 /H 2 /O 2 /Ar gas mixtures, allowed the estimation of the initial net rate of propylene epoxidation ( s -1 ). This reaction rate closely matched the TOF ( s -1 ) obtained for the Au-Ba/Ti-O at steady-state conditions. Ti-hydroperoxo species formed during propylene epoxidation conditions is a true reaction intermediate.